In modern internal combustion engines, devices for variably adjusting the control times of gas exchange valves are used in order to be able to design in a variable manner the phase relation between the crankshaft and the camshaft in a defined angular spread, between a maximum early and a maximum late position. For this purpose, the device is integrated into a drive train, via which torque is transmitted from the crankshaft to the camshaft. This drive train may, for example, be implemented as a belt drive, chain drive or gear drive.
A device of this type is known, for example, from U.S. Pat. No. 6,450,137 B2. The device comprises two rotors which are rotatable against one another, whereby an outer rotor has a drive connection with the crankshaft and the inner rotor has a rotatably fixed connection with the camshaft. The device comprises four pressure areas, whereby each of the pressure areas is divided by means of a vane into two counteracting pressure chambers. Through pressure medium delivery to the pressure chambers or pressure medium discharge from the chambers, the vanes are shifted within the pressure areas, whereby a targeted rotation of the rotors relative to one another and therefore of the camshaft relative to the crankshaft is effected.
The pressure medium delivery to and the pressure discharge from the pressure chambers are controlled by means of a control unit, normally a hydraulic directional valve (control valve). Emerging from the control valve, pressure medium lines are provided which open out into the respective pressure chambers.
The control unit in turn is controlled by means of a regulator, which, with the aid of sensors, determines and compares with one another the actual position and the required position of the phase angle of the camshaft in the internal combustion engine. If a difference between the two positions is detected, a signal is transmitted to the control unit, which adapts the pressure medium flows to the pressure chambers according to this signal.
In order to guarantee the function of the device, the pressure in the pressure medium circuit of the internal combustion engine must exceed a specific value. Since the pressure medium is normally provided by the oil pump of the internal combustion engine and the pressure provided therefore increases synchronously with the speed of the internal combustion engine, below a specific speed the oil pressure is still too low to change or maintain the phase angle of the rotors in a targeted manner. This may be the case, for example, during the start-up phase or idling phases of the internal combustion engine.
During these phases, the device would perform uncontrolled oscillations, resulting in increased noise emissions, increased wear, unsteady running and increased raw emissions of the internal combustion engine. To prevent this, a locking mechanism is provided which couples the two rotors in a mechanically rotatably fixed manner during the critical operating phases of the internal combustion engine.
The locking mechanism comprises two mechanisms limiting the angle of rotation, whereby a first mechanism limiting the angle of rotation, in the locked state, permits a displacement of the inner rotor relative to the outer rotor in an interval between a maximum late position and a defined mid-position (locking position). The second mechanism limiting the angle of rotation, in the locked state, allows a rotation of the inner rotor relative to the outer rotor in an interval between the mid-position and the maximum early position. If both limitations of the angle of rotation are in the locked state, the phase angle of the inner rotor relative to the outer rotor is limited to the mid-position.
Each of the mechanisms limiting the angle of rotation comprises a spring-loaded pin, which is disposed in a bore of the outer rotor. A force is applied by means of a spring to each of the pins in the direction of the inner rotor. A receiving member, which is located opposite the corresponding pin in specific operating positions of the devices, is formed on the inner rotor for each pin. In these operating positions, the spring-loaded pins can engage into the receiving member. The respective mechanism limiting the angle of rotation then transfers from the unlocked to the locked state.
Each of the mechanisms limiting the angle of rotation can be transferred from the locked to the unlocked state through the application of pressure medium to the receiving member. In this case, the pressure medium pushes the pin back into its receiving member, whereby the mechanical coupling between the inner rotor and the outer rotor is released. To guarantee the pressure medium delivery, each of the mechanisms limiting the angle of rotation is connected to one of the pressure medium lines. These lines run from the control valve to the respective receiving member of one of the mechanisms limiting the angle of rotation and on from there into the corresponding pressure chamber.
During the start-up process and the idling phases of the internal combustion engine, the two rotors are mechanically coupled by means of the mechanisms limiting the angle of rotation. However, the inner rotor performs a small-amplitude oscillating movement relative to the outer rotor. The cause of these oscillations is the locking clearance of the mechanisms limiting the angle of rotation which is required in order to enable secure locking of the pins into the receiving members, in conjunction with the oscillating torque which acts on the camshaft during the operation of the internal combustion engine. Due to the oscillating torques, the inner rotor is first rotated in a circumferential direction relative to the outer rotor, until this rotation is stopped by the one mechanism limiting the angle of rotation (first final position). A rotation in the opposite circumferential direction is then carried out until this rotation is stopped by the other mechanism limiting the angle of rotation (second final position). The phase angle difference between these final positions corresponds to an angle φ which is defined by the locking clearance. In the case of a movement from the first into the second final position, the volume of one of the two counteracting pressure chambers reduces by the amount V, while the volume of the other pressure chamber increases by the amount V. The following applies:
      V    =                  φ                  360          ⁢          °                    ·      π      ·      h      ·              (                              R            2                    -                      r            2                          )              ,where h is the axial length of the pressure chamber, R is the distance between the axis of rotation of the camshaft adjuster and the inner lateral surface of the pressure chamber and r is the distance between the axis of rotation of the camshaft adjuster and the outer lateral surface of the inner rotor next to the vanes.
If the pressure chambers are empty or not completely filled with pressure medium during the idling phases or the start-up phase, the oscillation of the inner rotor relative to the outer rotor could cause a pump effect. This pump effect may transport pressure medium into one or more pressure chambers. As a result, one of more pressure chambers may fill up completely with pressure medium without the pressure medium pump delivering sufficient system pressure to operate the device in a functionally reliable manner, i.e. to securely hold or specifically adjust phase angles.
If the volume of a completely filled pressure chamber is reduced by the amount V due to the oscillations, pressure peaks occur in the pressure medium system which are sufficient to force one or both pins of the mechanisms limiting the angle of rotation out of the receiving member back into the bores thereof. The mechanical coupling of the device is thus released at a time when the system pressure is not sufficient to fix or set the phase angle of the device (for example in the start-up or idle running phases). This results in large-amplitude oscillations of the inner rotor relative to the outer rotor in a circumferential direction of the device, as a result of which the exhaust gas behavior of the internal combustion engine is negatively influenced and, in the worst case, its start-up capability is not guaranteed.
A further device is known from U.S. Pat. No. 6,684,835 B2. The only difference here is a receiving member for the pins of both mechanisms limiting the angle of rotation. Furthermore, pressure medium is applied to the receiving member via a connection line, which is formed separately from the pressure medium lines which connect the control valve to the pressure chambers. The connection line communicates on the one hand with the receiving member and, on the other hand, with a control valve connection.
In this design form also, pressure peaks are generated in the device which may be propagated via the control valve through to the receiving member and may result in the same problems.